1. Field of the Invention
The present invention relates to the heating of the underground by ohmic dissipation of energy as current flows through the ground. More specifically, it relates to a method using electrical resistance tomography to adjust the distribution of energy deposition so that heating of a region underground is controlled by depositing energy in a predetermined pattern.
2. Description of Related Art
Many applications require heating of regions underground. For example, heat can be used to: (1) reduce the viscosity of heavy oil deposits so they can flow to a wellbore and be pumped to the surface; (2) vaporize contaminants in the soil so they can be collected in wells or the surface and removed from the ground; and (3) enhance the in situ microbial degradation of organic pollutants such as volatile organic compounds (VOC). For example, the growth and metabolic rates of many naturally occurring microorganisms double with each 5 to 10 degree Celsius increase in temperature between 20 and 45 degrees Celsius. Electric heating might therefore reduce the time required for microbial in situ remediation more than an order of magnitude with a possible commensurate cost savings. Also, temperature optimization generally results in more complete metabolic degradation, yielding lower residual pollution concentrations.
Joule heating of highly viscous oils to reduce viscosity and enable recovery of these resources is already being practiced in industry. The use of imaging techniques such as seismic tomography and nuclear magnetic resonance are also being developed by other groups. Any practical method to control the power dissipation pattern in the ground must start with information about the resistivity distribution in the region to be heated. To obtain this information requires an imaging method that is designed specifically for resistivity. Methods such as seismic tomography are not candidates. Seismic tomography images mechanical properties such as sound wave speed. A method that images the resistivity distribution itself is clearly to be preferred. Such a method is electrical resistance tomography. Electrical resistivity tomography (ERT) uses electric currents to image the region which is also being heated by electric currents.
Two problems exist with traditional subsurface heating methods such as hot air or steam injection. First, it is difficult to determine where heat is being deposited. Second, there is little or no control over where the heat is deposited. The proposed method is a new and novel approach to optimized heating-monitoring of the subsurface. Because of its unique capabilities, various underground processes would be practical which before were not: (1) heating heavy oil formations, (2) enhancing production of a depleted oil formation, (3) enhancing microbial metabolism of hydrocarbons and (4) volatilization of volatile organic compounds (VOC's) at a hazardous waste site.
Several methods have been used to heat the subsurface. These include injecting into the ground hot water, hot air, steam, electric current and even radio waves. Each approach has unique features. However, they all have in common that it is difficult to determine or control what is heated. For example, when a fluid such as steam is injected, it establishes a channel along the path of greatest fluid permeability, the most highly conducting route between the injection and extraction points. The more steam that flows along this path the higher the fluid permeability becomes and the lower the resistance to flow. Heat can be deposited along that path. However, there is no good method to determine the location of that path. Even more difficult, there is no way to change that path. Usually it is desirable to heat more than the zone chosen by nature. A way is needed to determine where heat is being deposited, and just as importantly, to change it at will. The present invention provides such a method.